Charge pumps are an essential power conversion building block that finds wide application in mobile electronic devices. Charge pumps are key parts of display driver ICs, LED backlight drivers and flash memory. For many rich-media mobile devices, the display and backlight driver can dominate the power consumption; often more than half of the average power is spent through charge pumps. Improving the efficiency of charge pumps has been a semiconductor industry focus for many years given its direct impact on battery life of mobile devices.
Charge pumps constitute a class of DC-DC converters that use capacitors as the core energy conduction and storage medium; another popular class of DC-DC conversion relies on inductive components. DC-DC conversion with inductive components is ideal for many applications and can reach very high efficiency at high loads; however, these converters tend to do less well (efficiency, cost, size) at light loading. Light loads generally will require larger, more expensive inductors; in contrast, lighter load applications require only smaller and lower cost capacitors. For some light load applications (e.g. displays), charge pumps are more cost-effective, smaller and easier to design than inductive DC-DC converters.
Charge pumps that multiply a source voltage by integer or fractional values are well known in the art. With available technology, unregulated multiplying charge pumps can be made exceptionally efficient (>99%) in certain regimes of operation. However, if the source voltage is unregulated, the output will not be regulated either.
The voltage and effective impedance of a battery can vary substantially with its charge state. To create a low impedance power supply voltage from a battery, the combination of a charge pump and voltage regulator is often used. In some applications, the battery voltage is first regulated and then multiplied; in other applications the battery voltage is multiplied and then regulated. In still other implementations, the multiplier is run at a varying clock rate to control the effective charge (or current) delivered to a load; as part of a closed loop regulation, the charge pump can be seen as an ideal voltage source at the open circuit voltage prescribed by the topology of the multiplier with a variable output impedance (proportional to clock rate) that can be made part of a feedback loop to achieve regulation.
In most of the charge pump and regulator combinations, there is a linear regulator function somewhere in the design. Linear regulation has the downside of dissipating power (and dropping voltage); the unavoidable dissipation is equal to the current supplied by the regulator times the voltage drop across the regulator. It is well known in the art to take the approach of choosing the lowest input voltage possible to minimize the losses due to voltage drop across a linear regulator.
In many display charge pump applications, a number of different voltages are required (e.g. 6 or more to control the gate, source and COM signals of a TFT LCD). To reduce cost and board area the number of charge pumps and the components are minimized to save cost and by sharing functions (e.g. make a single charge pump which can deliver multiple output voltages). Often the accuracy requirements for the voltages are not the same (e.g. COM voltages require finer tuning than gate signals, for example).
The function of regulation is very similar to amplification; a negative feedback loop from the output comparing the signal to a stable reference generates a correction signal to the output driver. Open loop charge pumps often suffer from high output impedance or excessive clock frequencies (which lead to higher quiescent power) to keep impedance reasonable. Negative feedback loops can be added to a natively high impedance output source (such as a slowly clocked charge pump) to achieve a much lower output impedance and can be thus considered as an avenue for power savings by then enabling the clock frequency of the charge pump to be reduced significantly. Analog amplification with an operational amplifier will typically consume some static power; hence a tradeoff exists between charge pump clocking frequency and amplifier power. If amplifier power. If the reduction in charge pump clocking power is much larger than the increase due to the addition of an amplifier, then the overall solution can reach a lower power while meeting similar (if not better) specifications for regulation (e.g. output impedance, stability, reliability, insensitivity to manufacturing variations, etc.) since closed loop regulation is generally consider superior to open loop regulation. If the amplification power costs is low enough, there are a host of benefits from closing the loop.
For regulating high voltages with a high voltage amplifier, some quiescent bias current is typically drawn from high voltage power rails in the output stage of a high voltage amplifier. Amplifiers constructed from high voltage devices tend to be large, slow and inefficient, often requiring more bias current to offset the device limitations. Overall it is difficult to build a high voltage amplifier with good characteristics and low power, compared to low voltage amplifiers where the devices and costs are much lower. And if the voltage supply rails of the operational amplifier are significantly different from the output voltage, such an amplifier will incur the same power loss as that seen in linear regulators (equal to the voltage drop times the sourced current) which will negatively impact the efficiency. Any approach that includes negative feedback regulation of high voltages needs to consider such issues.
Analog amplifiers are broadly categorized into classes—a Class AB, for example, is a push-pull output configuration. To achieve greater efficiency, efficient DC-DC power supply ideas have been adopted by amplifier designers. Many of these advanced high efficiency amplifiers make use of inductors, at sometimes significant expense and size increase compared to the linear (e.g. Class AB) equivalent. Examples are Class D (inductor in the output) or Class H (continuously variable output supply rails from a typically inductive based DC-AC converter). Class G amplification (multi-rail) is also a possible architecture, but is often prohibitively complex (more pins, components, board area and quiescent current from HV sources) that it negates the power advantage of closed loop & low frequency charge pumps. To avoid inductors (which have difficulty reaching high efficiency, low cost and small board space at light power loads) a new topology is needed that integrates advanced amplification (e.g. a variable rail voltage or multi-rail) with charge pumps without incurring the disadvantages of the common approaches available to those skilled in the art.
The charge pump and regulation topology described herein addresses the limitations of conventional combined charge pump and regulation circuits by using a single low voltage operational amplifier to provide closed loop low impedance accurate voltage generation for a number of output voltages using a high efficiency “class G” amplifier topology integrated with a Dickson charge pump.